At first blush it looks identical to the 700K, except for the color of the backplane. It has the same layout, the same features, and the same styling, so why is this the "500K"? Because it's not a mechanical keyboard, but rather a membrane keyboard with rubber dome switches.
As we mentioned, it otherwise has the same features as the 700K. It comes with N-Key rollover (which the company points out is a rarity among membrane keyboards), six programmable G-Keys (one of which is half of the space bar), full LED lighting, media control switches, macro recording controls, and windows-key lock button.
This keyboard is for the users who really liked what they saw with the 700K but didn't feel like forking over the hefty price tag that comes with mechanical keyboards. Mechanical keyboards may offer a superior typing experience, but if you haven't typed on one long enough to get addicted yet, you can choose to save some dough and get this keyboard instead.
Cougar priced the 500K at $74.99 and it will be available Q1 2015.
UPDATE: This article was updated at 9:22 EST, December 12 2014 to add pricing information and to remove the box art, which incorrectly said "Mechanical Gaming Keyboard".